Process for the polymerization and copolymerization of ethylene, using a gas injector device

ABSTRACT

Process and device for cooling a mixture of polymer and monomer from a reactor in which ethylene is polymerized or copolymerized at a pressure greater than 1,000 bars between a pressure-reduction valve located downstream of the reactor and a medium pressure separator. The cooling is achieved by injecting monomer at a pressure below that of the separator using a device with a nozzle distance S 1  and corresponding cross-section A 1  the mixture from the reactor passes, a convergent mixing zone in which the mixture is mixed with the monomer injected, which is supplied at a flow rate q, and a diffuser of throat distance S 3  and corresponding cross-section A 3 , which makes it possible to bring the mixture to the pressure of the separator. The dimensions of the device are such that the ratio Q/A 1  is between 0.20 and 1.35 t/hr.mm 2  and the ratio ##EQU1## is between 0.1 and 0.3 t/hr.mm 2 .

This is a continuation of application Ser. No. 112,606, filed Jan. 16,1980, abandoned, which is a division of Ser. No. 692,380, filed June 3,1976, now U.S. Pat. No. 4,215,207.

The present invention relates to an improved process for thepolymerisation and copolymerisation of ethylene under a pressure greaterthan 1,000 bars. More precisely, it relates to a process which makes itpossible to cool the mixture of polymer and monomer(s) coming from thereactor before it enters the medium pressure separator, as well as to adevice for carrying out this process.

In the conventional scheme of manufacture of polyethylene under apressure greater than 1,000 bars, the mixture of ethylene andpolyethylene coming from the reactor undergoes a reduction in pressurein a valve before being passed into a separator which operates at amedium pressure generally between 200 and 500 bars. This reduction inpressure is accompanied by a rise in temperature which can exceed 20°C.; modifications which are detrimental to the properties of the polymermay take place in the separator if its temperature is too high. To avoidthese undesirable modifications, which can go as far as degradation inthe case of ethylene/vinyl acetate copolymers, it is advantageous tocarry out the separation at a temperature below that which results fromthe reduction in pressure of the mixture between the reactor and theseparator. This problem can be resolved in at least two ways; a firstmethod consists in interposing an exchanger between the reactor and theseparator, but firstly this solution entails expensive investment andsecondly it presents additional safety problems because of the risk ofplugging by the polymer. A second method consists in injecting freshand/or recycled ethylene, at a pressure greater than that of theseparator, between the reactor and the separator; the gas to beinjectecd can be taken from the primary compressor output in the case offresh gas, or downstream from an exchanger of the recycling circuit inthe case of gas recycled under medium pressure (200 to 500 bars).

This latter method is used in British Pat. No. 1,338,280 according towhich on injecting, at the pressure reducing valve loated downstreamfrom a tubular reactor, a mixture of fresh ethylene and recycledethylene under a pressure of 200 to 350 bars, the optical properties ofthe resin are improved whilst the length of the reactor is reduced.Also, U.S. Pat. No. 3,509,115 proposes, in the case of ethylene/vinylacetate copolymers, to introduce feed ethylene under a pressure greaterthan that of the separator. The injection at a pressure greater thanthat of the separator requires either increasing the output pressure ofthe primary compressor or installing a supplementary compressor in thegas injection circuit. This action entails both new investment and anincrease in the operating costs.

In contrast, the process according to the invention, which avoids thevarious abovementioned disadvantages, is characterised in that thecooling of the mixture coming from the reactor is achieved by theinjection of ethylene, at a pressure below that of the separator,between the pressure reducing valve and the separator. It is applicable,and advantageous, both in the case of an autoclave reactor and in thecase of a tubular reactor, in the case of a radical polymerisation usinginitiators of the peroxide or perester type, as well as in the case ofan ionic polymerisation employing catalysts of the Ziegler type. It isapplicable to a great variety of ethylene copolymers, in which thecomonomer(s) may in particular be maleic anhydride, vinyl acetate,propylene and carbon monoxide, and can be present in concentration of upto 20% by weight. For this reason the words monomer and ethylene will,in the text which follows, denote both ethylene alone and mixtures ofethylene with other comonomers.

Further objects of the invention are to provide an apparatus forcarrying out the process defined above and to define an injection devicewhich makes it possible to draw the monomer at a pressure of the orderof 50 bars. In fact, in the case of a number of industrial units for themanufacture of polyethylene under high pressure, which are supplied withthe ethylene from the refinery under a pressure close to the criticalpressure (51 bars), the possibility of introducing the fresh monomer bymeans of this device emerges as an essential advantage of the invention,which permits a reduction in the capacity--or even the completeomission--of the primary compressor, the usual role of which is to bringthis monomer to a pressure of about 150 to 450 bars. It is evenpossible, by coupling several devices according to the invention, todraw this monomer at a pressure markedly below 50 bars but at a lowerflow rate. The industrial value of the invention thus resides in thereduction of the investment costs and operating costs of the unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of one embodiment of the invention.

FIG. 2 is a flow diagram of a second embodiment of the invention.

FIG. 3 is a schematic representation of the gas injection device of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the presently preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. The drawings include exemplary flow rates,pressures, and compositions.

FIG. 1 represents one embodiment of the process according to theinvention, in which the injected gas comes from an exchanger 1 of therecycling circuit after having been reduced in pressure through a valve2.

FIG. 2 represents another embodiment in which the injected gas comesfrom a distribution circuit under a pressure of the order of 50 bars.

FIG. 3 represents an injection device--or injector--which is generallycomposed of three parts:

an ejection nozzle of throat distance S₁ and corresponding cross-sectionA₁, the purpose of which is to impart a certain speed to the fluidcoming from the reactor at a flow rate Q, this fluid being referred toas the motive fluid;

a mixing zone which makes it possible to draw the flow rate q of freshor recycled monomer, referred to as drawn fluid, with the motive fluid;and

a diffuser of throat distance S₃ and corresponding cross-section A₃ thefunction of which is, by reducing the speed of the resulting mixture, toachieve a sufficient pressure at the injector outlet.

The nozzle is part of the inlet jet of the injection body whilst the twoother parts belong to the outlet jet of same body. These pieces arepreferably made of a treated steel which has a tensile strength of 100to 160 kg/mm2.

The device according to the invention is characterised by a ratio Q/A₁of between 0.20 and 1.35 t/hr.mm2 and by a ratio ##EQU2## of between0.10 and 0.30 t/hr.mm2.

It operates as follows: the fluid issuing from the reactor is reduced inpressure at the extrusion valve and along the pipeline leading to theseparator, and reaches a pressure P₀ upstream from the nozzle (distanceS₀ and corresponding cross-section A₀).

The nozzle has a geometry such that at its throat (cross-section A₁) themotive fluid has reached the speed of sound which causes it to flowunder so-called critical conditions. The combination of mixing zone anddiffuser can be compared to a converging-diverging system of specialcross-sections. The transfer of the momentums between the motive fluidand the drawn fluid takes place in the converging section and the speedof the two fluids is identical at the end of the mixing zone(cross-section A₃). Finally, the speed of the mixture is converted topressure in a quasi-isentropic manner in the diverging section.Throughout all these operations some energy, lost through eddies andfriction, is converted into heat. One of the essential characteristicsof the operation lies in the fact that the pressure at the outlet of thediffuser is regulated, because it is equal to the pressure of theseparator, increased by the drop in pressure between the diffuser outletand the separator.

The following facts allow a better understanding of the concept of thedevice and of the requirements regarding each of its components. Becauseof operating under critical conditions, the pressure P₀ only depends onthe flow rate Q of fluid passing through the nozzle and not on thereactor pressure. On the other hand, in order to reduce the injectionpressure P₂ and/or increase the flow rate q, it is appropriate toincrease the momentum of the motive fluid as it leaves the nozzle; inorder to do this, it is appropriate either to fit a diverging sectiondownstream from S₁ or to reduce S₁, but this latter solution is limitedby the need of not interfering with the pressurer regulation of thereactor. It was thus necessary to know the relation between P₀, Q andA₁, but this could not be correctly predicted by calculation becauseprecise data relating to the behaviour of the monomer-polymer mixtureunder such conditions were lacking. We have found experimentally thatthe ratio ##EQU3## must be between 0.20 and 1.35 t/hr.mm2 for pressuresP₀ of about 400 to 2,500 bars, as is shown by Table I below.

                  TABLE I                                                         ______________________________________                                        R(t/hr·mm2)                                                                   1.34   1.09   0.84 0.59 0.49 0.39 0.33 0.24                          ______________________________________                                        P.sub.o (bars)                                                                         2,500  2,000  1,500                                                                              1,000                                                                              800  600  500  400                           ______________________________________                                    

The other abovementioned solution for increasing the momentum of themotive fluid consists in fitting a diverging section of outlet distanceS₁ and corresponding cross-section A'₁ downstream from the throat of thenozzle, as indicated in broken lines in FIG. 1, the ratio A'₁ /A₁ of thecross-section being preferably between 1 and 1.5.

In the mixing zone, the transfer of a part of the momentum of the motivefluid to the drawn fluid takes place with a certain yield. There again,predicting a correlation between the admission pressure P₂ of theinjected gas nad its flow rate q is impossible because firstly themixing of the two fluids takes place gradually in a zone of variablecross-section and not entirely in a zone of constant cross-section,secondly the resultant of the forces exerted by the fluids on the wallsis not zero, and finally, the relatively low pressures (less than 200bars) encountered in this zone cause the demixing of a phase which isrich in monomer and a phase which is rich in polymer. Accordingly, theconventional models of the mechanics of fluids do not provide thecorrect solution to this problem. In order to lower the suction pressureP₂, it is possible to reduce the momentum of the mixture by reducing thecross-section A₃ of the throat of the diffuser, the minimum valuecorresponding to operation at the speed of sound.

As it has just been explained in connection with the nozzle and themixing zone, the geometry of the diffuser is also subject to certainrequirements. In fact, on reducing the cross-section A₃ until criticalconditions are achieved, a supersonic flow is created in the divergingsection. This necessarily results in a shock wave located at across-section upstream A₄ corresponding to distance S₄, so that thefluid is recompressed up to the pressure of the separator. If A₃ isfurther reduced, the flow will remain supersonic in a part of thediverging section, but the pressure upstream from the throat willincrease. Consequently, the cross-section A₃ must not be less than acertain minimum value so as to ensure against any pressure rise in themixing zone. Thus it has been found experimentally that the optimumvalue of ##EQU4## was between 0.1 and 0.3 t/hr.mm2 and preferablybetween 0.16 and 0.24 t/hr.mm2; this ratio B is a parameter which isclosely related to the efficiency of the injector.

It may be recalled that for the construction of this type of apparatusit is known to choose an apex half-angle of between 2°30' and 5° bothfor the nozzle and for the diffuser. Secondly, though the temperature ofthe fluid injected is not characteristic of the process, it is obviousthat cooling the mixture issuing from the reactor will be the moreefficient, the lower this temperature is. In practice, usualtemperatures for the introduction of this monomer are between -20° C.and +120° C. Finally, the cross-sections can be chosen with anygeometrical shape, but it will be found advantageous to choose, for theoutlet cross-section of the nozzle, A₁ or A'₁ depending on the case, ashape which makes it possible to achieve the largest possible contactsurface between the two fluids.

The examples which follow are intended to illustrate the invention butmust not be interpreted as limiting it.

EXAMPLE 1

We shall consider an installation for the radical polymerisation ofethylene, comprising a stirred autoclave reactor operating under apressure of 1,600 bars, at the outlet of which the temperature is 270°C. In the absence of an injector according to the invention, thetemperature of the separator, which operates under 265 bars, is 290° C.

An injector comprising a nozzle of length 68 mm and circularcross-section, with diameters φ₀ =21 mm and φ₁ =15 mm, a mixing zone oflength 32 mm and diameter φ₂ =44 mm, and a diffuser having an apexhalf-angle of 4°, and defined by the diameters φ₃ =25 mm and φ₄ =35 mm,is fitted onto this installation.

A series of experiments was carried out, varying the ratio q/Q whilstmaintaining the total flow rate q+Q passing through the diffuserapproximately constant. Under these conditions, the pressure R₀ upstreamfrom the nozzle is about 500 bars, and for each experiment the injectionpressure P₂, which will be found in Table II below, is determined.

                  TABLE II                                                        ______________________________________                                        Experiment No.                                                                             1       2         3     4                                        ______________________________________                                        Q (t/hr)     63.8    63.1      61.5   56                                      q (t/hr)     1.25    3         5      9                                       P.sub.2 (bars)                                                                             140     160       170   180                                      ______________________________________                                    

The ratio R varies, according to the experiments, from 0.32 to 0.36t/hr.mm2 and the ratio B is 0.133 t/hr.mm2. Furthermore, the ethylenebeing injected at 25° C., the temperature measured at the separator is235° C. for experiment No. 4 and 280° C. for experiment No. 1.

This shows that such an injector allows appreciable cooling of themixture coming from the reactor whilst maintaining the injectionpressure P₂ at a level below that previously known. This reduction in P₂induces an advantageous reduction of the costs of operating the process.

EXAMPLE 2

We shall consider a polymerisation installation of the same type as inExample 1, operating under identical conditions. Another injectorcomprising a nozzle with diameters φ₀ =15 mm and φ₁ =8.5 mm, a mixingzone of length 21 mm and diameter φ₂ =27 mm, and a diffuser withdiameters φ₃ =12.5 mm and φ₄ =32 mm is fitted onto this installation.

Two experiments are carried out, keeping the flow rate Q of the motivefluid constant and equal to 23.6 t/hr; under these conditions, thepressure P₀ upstream from the nozzle is 650 bars and the coefficient Ris 0.41 t/hr.mm2. The results of the two experiments are summarised inTable III.

                  TABLE III                                                       ______________________________________                                        q (t/hr)      P.sub.2 (bars)                                                                         B (t/hr · mm.sup.2)                           ______________________________________                                        0.8            85      0.199                                                  4.9           110      0.232                                                  ______________________________________                                    

We claim:
 1. An apparatus for the polymerization or copolymerization ofethylene in the absence of a compressor used to raise the pressure ofmonomer to the level of a separator, said apparatus consistingessentially of:(a) a reactor for polymerizing or copolymerizing saidethylene; (b) an expansion valve connected to the outlet of said reactorfor reducing the pressure of reaction mixture from said reactor; (c) agas injection device connected to the outlet of said expansion valve,said gas injection device comprising at least one assembly comprising anejection nozzle having a progressively decreasing diameter in thedownstream direction with a throat of cross-section A₁ at the downstreamend of said nozzle, through which throat arrives said reaction mixturefrom said expansion valve, converging means defining a zone ofprogressively decreasing diameter in which said reaction mixture ismixed with a gaseous monomer, the upstream end of said means beingconcentric with, extending to the downstream end of, and having a largerdiameter than the downstream end of said ejection nozzle, and a diffuserhaving (1) a throat of cross-section A₃, which diffuser throat islocated at and connects the upstream end of said diffuser and thedownstream end of said converging means, for bringing the mixture ofreaction mixture and monomer to the pressure of said separator, saidcross-section A₃ being the same as the cross-section of the downstreamend of said converging means, and (2) means defining a zone ofprogressively increasing diameter whose upstream end is located at andhas the same cross-section as said diffuser throat and whose downstreamend constitutes the outlet of said device; (d) a line connected to saidconverging means at its upstream end for supplying said gaseous monomerat a temperature lower than that of said reaction mixture and at apressure below that of said separator, the upstream end of saidconverging means being open to fluid flow from only said ejection nozzleand the line; (e) means for introducing monomer, at a temperature lowerthan that of said reaction mixture and at a pressure below that of saidseparator, into the line of paragraph (d); (f) a separator, theseparator being connected to the outlet of said gas injection device,said separator being a means for separating polymer and gaseous monomerin said mixture; (g) a compressor connected to the outlet of saidseparator, said compressor being a means for compressing gaseous monomerseparated from said mixture in said separator to the pressure of saidreactor; (h) a line connected between said compressor and said reactor,said line being a means for carrying said compressed gaseous monomerfrom the outlet of said compressor to said reactor; and (i) a polymerdischarge line connected to said separator.
 2. Apparatus according toclaim 1, wherein the ejection nozzle is provided downstream from thethroat of cross-section A₁, with a divergent zone of outletcross-section A'₁, the ratio A'₁ /A₁ being between 1 and 1.5.